powerpc/opal: Add missing include

next-20140324 currently fails compiling celleb_defconfig with:

arch/powerpc/include/asm/opal.h:894:42: error: 'struct notifier_block' declared inside parameter list [-Werror]
arch/powerpc/include/asm/opal.h:894:42: error: its scope is only this definition or declaration, which is probably not what you want [-Werror]
arch/powerpc/include/asm/opal.h:896:14: error: 'struct notifier_block' declared inside parameter list [-Werror]

This is due to a missing include which is added here.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>